In the context of manufacturing blades for wind turbines, the course of manufacture is that laminate blade shells are moulded in moulds and are subsequently assembled to form a complete blade. In connection with the moulding process, a number of root bushings with threads are arranged at the root of the blade for use in connection with mounting on the hub of the wind turbine by means of bolts. In the context of this, it is important that the root bushings terminate in the same root bushing plane and, at the same time, it is important that the plane in which the root bushing plane is situated is the correct plane relative to the blade, since otherwise it may mean that the blade is mounted askew and that the bearing between hub and blade is thereby forced askew. If all the root bushings are not in the same root bushing plane, it may mean that some of the bushings are caused to carry more load than others, thereby considerably deteriorating the longevity of the bearing and hence of the wind turbine.
Albeit tools are known intended for ensuring that the root bushings are plane in connection with the moulding integrally in the root of the blade shells, the shells deform and they shrink in connection with the curing of the laminate material, which means that the root bushings can be caused to be askew. This also means that it is impossible to predict the skewness the root bushings end up with simultaneously with the skewness differing from one blade to another.
Today, the above issue is solved by the root bushings being grinded following finished moulding, and this procedure takes place by the finished blade resting on a support, eg a blade carriage, following discharge from the mould, following which a cylindrical grinding machine is mounted in the root of the blade which grinds the root bushings and thereby shortens some of the root bushings to the effect that they are caused to terminate in the same root bushing plane. The grinding procedure as such takes place by a person monitoring spark formation from the root bushings, and when sparks are generated at all root bushings it is due to the fact that all bushings terminate in a common root bushing plane that corresponds to the grinding plane.
The problem of the above method is that, following discharge from the mould, the blade yields and deforms due to the self-weight of the blade, and therefore the plane grinding relative to the blade is no longer accurate and homogeneous. This means that the root bushing plane may be askew relative to the blade and the desired plane, and moreover the root bushing plane does not become uniformly plane either for blades that are to partake in the same wind turbine. In connection with the leveling of the root bushing plane there is often, in particular when large blades of about 50 m or more are concerned, a very little tolerance in the surface evenness in the order of ±2/10 mm. Moreover a subsequent verification of the root bushing plane is problematic as it is performed in relation to points on the blade.